1. Field of the Invention
The invention relates to a control apparatus, a control method, and a computer-readable storage medium for a continuously variable transmission. More specifically, the invention relates to a technology in which a belt clamping force is optimally controlled in a range in which a transmission belt does not slip.
2. Description of the Related Art
A continuously variable transmission for a vehicle, which includes a continuously variable transmission mechanism, is available. The continuously variable transmission transmits power using a transmission belt wound on a primary pulley and a secondary pulley. The speed ratio of the transmission mechanism is continuously changed by changing effective diameters of the primary pulley and the secondary pulley. When employing the continuously variable transmission, because the speed ratio is continuously changed, it is possible to operate an engine so that an operating point of the engine is as close as possible to an optimum fuel efficiency curve.
In the continuously variable transmission, if a belt clamping force for clamping the transmission is high, torque transmission efficiency is low. Therefore, it is conceivable to decrease the belt clamping force for clamping the transmission belt. However, if the belt clamping force for clamping the transmission belt is decreased, the transmission belt slips on the primary pulley and/or the secondary pulley. As a result, the primary pulley, the secondary pulley, and the transmission belt may be worn, and the durability of the continuously variable transmission may be decreased. Therefore, the slip of the transmission belt is prevented by increasing the belt clamping force by a predetermined margin.
For example, if a vehicle provided with the above-described continuously variable transmission starts moving on an upward slope, and torque input to the continuously variable transmission is increased, the transmission belt may slip. For example, Japanese Patent Application Publication No. 2001-263473 (JP-A-2001-263473) describes a hydraulic pressure control apparatus for a continuously variable transmission, which is a technology for preventing the slip of the transmission belt. When a vehicle speed is low, and it is difficult to detect an output rotational speed, a shift control valve is opened, and thus, the belt clamping force for clamping the transmission belt becomes unstable. As a result, the transmission belt may slip on the pulley(s). Accordingly, when the vehicle speed is low, the hydraulic pressure control apparatus controls a hydraulic pressure based on a low-speed line pressure characteristic, to prevent the slip of the transmission belt. When the low-speed pressure characteristic is used, the line pressure is set to a value higher than a value set when a line pressure characteristic for normal traveling is used.
The hydraulic control apparatus for a continuously variable transmission described in the publication No. 2001-263473 controls the line pressure based on the vehicle speed. When the vehicle speed is low, the line pressure is increased, regardless of whether the input torque is increased and decreased while the vehicle is in a stopped state. That is, even if the belt is not likely to slip, the belt clamping force is increased as long as the vehicle speed is low. Therefore, a friction loss and a pump load are large.